Certain steels characterized as free-cutting steels or machinable steels are advantageous for machining purposes. A number of such steels are known and have different advantages relating to the particular machining process to be used, the size, the shape, and surface finish required commercially, the economy of operations, and the like. Such machinable steels include certain carbon steels, resulfurized carbon steels, rephosphorized carbon steels, leaded carbon steel, carburizing steels, alloy carburizing steels, and certain hardenable alloy steels.
During the early 1960's, free machining steels which exhibited superior internal quality in thicknesses up to twelve inches, in comparison with plain resulfurized free-machining steels, were developed by using special lead addition equipment, by controlling sulfur levels and, in some cases, by vacuum degassing the molten steel. However, in the early 1970's, environmental requirements which govern the production of leaded steels became highly restrictive and, as a result, the production of leaded steels became more difficult and considerably more expensive. A need thus arose for a substitute steel which could be produced without lead while maintaining the desired machinability and internal quality of the steel.
It is well-known that resulfurized steels have better machinability than steels of the same composition without the sulfur added to the steel. Table I shows twenty-two steels in the SAE 1100 series of free-cutting steels. Sulfur is added to such steels for the sole purpose of decreasing the machinability cost by increasing productivity through greater machining speeds and improved tool life.
TABLE I __________________________________________________________________________ FREE-CUTTING CARBON STEEL COMPOSITIONS SAE P (range NO. C Mn or max) S Si Pb __________________________________________________________________________ 1111 0.13 max. 0.60-0.90 0.07-0.12 0.08-0.15 1112 0.13 max. 0.70-1.00 0.07-0.12 0.16-0.23 1113 0.13 max. 0.70-1.00 0.07-0.12 0.24-0.33 12L14 0.15 max. 0.80-1.20 0.04-0.09 0.25-0.35 0.15-0.35 1108 0.80-0.13 0.50-0.80 0.040 0.08-0.13 0.10 max. 1109 0.08-0.13 0.60-0.90 0.040 0.08-0.13 0.10 max. 1115 0.13-0.18 0.60-0.90 0.040 0.80-0.13 as specified up to 0.30 1117 0.14-0.20 1.00-1.30 0.040 0.08-0.13 as specified up to 0.30 1118 0.14-0.20 1.30-1.60 0.040 0.08-0.13 as specified up to 0.30 1119 0.14-0.20 1.00-1.30 0.040 0.24-0.33 as specified up to 0.30 __________________________________________________________________________ SAE P NO. C Mn max. S Si __________________________________________________________________________ 1120 0.18-0.23 0.70-1.00 0.040 0.80-0.13 as specified up to 0.30 1126 0.23-0.29 0.70-1.00 0.040 0.80-0.13 as specified up to 0.30 1132 0.27-0.34 1.35-1.65 0.040 0.08-0.13 as specified up to 0.30 1137 0.32-0.39 1.35-1.65 0.040 0.08--0.13 as specified up to 0.30 1138 0.34-0.40 0.70-1.00 0.040 0.08-0.13 as specified up to 0.30 1139 0.35-0.43 1.35-1.65 0.040 0.12-0.20 as specified up to 0.30 1140 0.37-0.44 0.70-1.00 0.040 0.08-0.13 as specified up to 0.30 1141 0.37-0.45 1.35-1.65 0.040 0.08-0.13 as specified up to 0.30 1144 0.40-0.48 1.35-1.65 0.040 0.24-0.33 as specified up to 0.30 1145 0.42-0.49 0.70-1.00 0.040 0.04-0.07 as specified up to 0.30 1146 0.42-0.49 0.70-1.00 0.040 0.08-0.13 as specified up to 0.30 1151 0.48-0.55 0.70-1.00 0.040 0.08-0.13 as specified up to 0.30 __________________________________________________________________________
Table II discloses two alloy carburizing steels, SAE 4024 and 4028. It is known that better machining alloys can be obtained by the addition of sulfur. Two widely used alloy steels, SAE 4340 and 8640, may have their machinability improved, without comprising dimensional accuracy, through the addition of sulfur.
TABLE II ______________________________________ ALLOY CARBURIZING STEELS SAE NO. C Mn P S Si Mo ______________________________________ 4024 0.20-0.25 0.70-0.90 0.040 0.035-0.050 0.20-0.35 0.20- max. 0.30 4028 0.25-0.30 0.70-0.90 0.040 0.035-0.050 0.20-0.35 0.20- max. 0.30 ______________________________________
The larger sulfide inclusions in resulfurized steels tend to cause pitting and cracking on the machined surface, thus limiting their use. This is in contrast to leaded grades of steel wherein there is no significant or discernible difference in static strength, ductility or notch sensitivity between fine-grain leaded carbon steels and their non-leaded counterparts. Accordingly, if the starch for a substitute for leaded steels was to be found in resulfurized steels, then it was necessary to eliminate or minimize large sulfide inclusions in such resulfurized steels as may cause pitting and cracking.